Metham sodium (sodium methyldithiocarbamate, also know as metam sodium, metam and metham, and marketed under the names Metam, Sectagon II, Soil-Prep, Vapam and V.P.M.) is commercially available for use as a preplanting soil fumigant solution for plant crops. After application to the soil, the liquid solution is converted into a gaseous fumigant, methyl isothiocyanate, which is known to be effective as a fungicide, pesticide and herbicide for the control of weeds, germinating weed seeds and insects, such as Henbit, Lambsquarters, Pigweed, Careless weed, Watergrass, Johnsongrass, Nutgrass, Wild morning-glory, Purslane, Nematodes, Symphylids, and as a pesticide/fungicide for the control of soil-borne diseases, such as Rhizoctonia, Phythium, Phytophora, Vericillium, Sclerotinia, Oat root fungus and Club root of crucifers. In a 32.7% aqueous solution (3.18 pounds by dry weight of active ingredient per gallon), metham sodium is applied as a preplanting fumigant at application rates of 50-100 gallons (159-318 pounds by dry weight of active ingredient)per acre.
Due to its highly toxic nature, planting may not take place until 14-21 days after conventional metham sodium application for well-drained soils of light-to-medium texture, for 30 days after conventional treatment for soils that are heavy or especially high in organic matter (or that remain wet and/or cold), or until at least 60 days after conventional treatment where metam sodium dosages greater than 100 gallons per acre have been applied.
The fumigant and/or fungicidal activity of metham sodium and biologically active derivatives and analogs of metham sodium have been known in the art for some time. For example, U.S. Pat. No. 2,614,957 discloses a fungicidal composition containing about 75:99% by weight of a metham sodium analog, sodium dimethyldithiocarbamate, and about 25:1% by weight of the sodium salt of 2-mercaptobenzothiozol.
U.S. Pat. Nos. 2,614,959 and 776,922 disclose the fungicidal activity of compositions of the zinc salt of dimethyldithiocarbamate combined with the ferric or zinc salt of mercaptobenzothiozol.
U.S. Pat. No. 2,792,327 discloses a method of controlling fungus infestation by applying sodium monomethyl dithiocarbamate directly to the locus of the vegetation to be treated for fungus control. In practice, metham sodium is applied in a foliar application to plant leaves or as a preplant seed treatment. It is stated that the method can be used to treat living plants, such as crop plants, ornamental plants, fruit bearing trees, and plant seeds. Suitable application rates are stated to be about 1 to 4 pounds per acre for use as a foliar spray, about 0.5-3 oz. per 100 pounds of seed to be treated, or about 0.005 to 0.02 pounds of sodium monomethyl dithiocarbamate per pound of organic matter treated.
U.S. Pat. No. 2,835,625 discloses a fungicidal metham sodium derivative, isopropoxycarbonyl dimethyldithiocarbamate.
U.S. Pat. No. 3,699,231 discloses the use of a mixture of metham sodium and formaldehyde in a 1/1 weight ratio to inhibit the growth of bacteria, such as in drilling fluids.
In addition, metham sodium has been used in the art in connection with the preservation of harvested crops. For example, U.S. Pat. No. 3,836,655 discloses a composition of metham sodium and propionic acid for use in preserving moist grain during storage. U.S. Pat. No. 4,708,879 discloses the use of metham sodium for preserving high moisture forage crops, such as hay, to prevent microbial degradation during storage.
The use of metham sodium as a preplanting herbicide applied through sprinkler systems, low-volume drip irrigation systems, subsurface blading and as a drench, is described in detail in Fischer et al., "The Use of Metham For Weed Control--A Progress Report," Runcina, Vol. 41, University of California Cooperative Extension, Fresno County, pp. 1-17 (1989). Its use as a herbicide, fungicide and nematocide has continued essentially unchanged for at least the last 30 years. While the soil fumigant and fungicidal properties of metham sodium are well known in the art, its highly toxic nature presents difficulties when used in connection with crop planting for fungicidal purposes. For example, as conventionally applied, a significant amount of time ranging from about 7 days to about 60 days must elapse prior to crop planting in order to prevent metham sodium damage to the newly planted crop. This results not only in significant time delays between fungicide application and crop planting, but also requires multiple pass working of the crop field. In addition, the relatively high cost of metham sodium makes conventional application prohibitively expensive for marginal application conditions.